Ah. What makes foam so wonderful as a prop material is also its downfall. It can be damaged or even snapped with far more easy than plastic or wood. So, what do you do? I'll run you thought the basics of what I personally do to protect my foam, followed by other suggestions.
1) First layer - gesso. Makes it so resin or paint won't soak into the foam, also begins to smooth out places in the foam I can't get sand paper into or have had trouble.
2) Second layer - sealant. If you plan to use a non-foam specific resin, you will need a layer of sealant on the foam. Cheapest way to do so? Elmer's White Glue. Just paint it on with a foam brush, being very, very careful to cover every inch of that foam. I still would recommend doing gesso layers as well. I find you can never be too safe.
3) Third Layer - resin or bondo. Crystal Sheen
(info credit: Pocketfightr) is a product I have personally used and continue to use. Unlike many other resins, this does not melt the foam even if put on unprotected foam.
This resin is crystal clear when dried. It is strong, but does not have fiberglass resin's strength. Dries over night. 1 to 1 ratio, incredibly easy to use. Very little smell or fumes to deal with. High recommended. Does not sand by hand easily, had to use power tools to help. Still can be pressed inwards on foam even after drying, but is very good protection. THIS
prop took the majority of a 32 ounce bottle (meaning one 16 ounce hardener and one 16 ounce resin bottle) to completely cover, including the scope. I had about an inch in the bottle leftover.
Fiberglass resin is able to be used on foam, but not without sealing it first. This stuff is damn near impervious when dried and hardens to a strong plastic like finish. This stuff is highly dangerous to work with on several levels - it will burn skin if it touches you while it's still hardening, gets very hot while hardening, and has serious fumes to work around. You will need a heavy duty face mask for this, latex gloves, and body coverage to make sure it doesn't land on you. Basically a trade off - this stuff is far more protective of your prop than Crystal Sheen, but can melt the foam if you aren't careful and you.
Bondo is the choice I will put last for several reasons. While it will not melt your foam, this stuff is a) expensive in larger amounts and b) very heavy very quickly. Resin will remain fairly light even in bulk, but bondo becomes heavy quickly. Bondo can be found in Lowes, Home Depot, etc in the same place as resin. It is used to repair car cracks. Like fiberglass resin, once dried, bondo is practically unbreakable.
(Extra notes dealing with resin:
You will 100% need to work outside when using resin. This stuff gets absolutely everywhere. Everything sticks to it. If you leave it outside to dry, be prepared to deal with little buggy bodies probably stuck to it. Do not leave something drying on a material where it is touching and it will forever stick to it, like the floor of your house. Liquid resin can be pulled off your skin by use of acetone (nail polish remover!) while it is still liquid. Once hardened on clothing/floor/etc, you will need to scrape it off or use a much stronger acid to get it off/out. Good luck at that point. Also, be aware you will end up having to sand off drips of resin. I've never been able to avoid that.)
4) Fourth layer - paint job. Obvious here - paint your prop. Be aware that if you use resin, you will want to take sand paper to that resin first to get the surface rough before you paint. I use basic, cheap acrylic paints, but pretty much anything can be used at this point.
5) Fifth layer - varnish. I use just a normal old wood varnish on the outside of my props. This is just to protect the paint job from damage.
1) Be very careful with your prop until it is sealed in resin or bondo!
This foam is dense but snaps very easily!
2) It is also flammable! Keep away from open fire. I called 1800-Get-Pink and they said a hot knife/hot wire cutter will melt the foam, but not make it catch fire. If you leave it near a fireplace/candle/stove/whatever, it DOES have a flash point! So do not –sand- or –cut- near an open flame, because the dust can ‘flash’ and make a fireball. The foam-dust does not travel all that far, but better safe then sorry!
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, or IM me at MikkouKoneko if you have any other questions!